Tuesday, November 6, 2007 - 10:00 AM

Penman-Monteith Equation Coefficients for Greenhouse Tomatoes.

Allan Matthias1, Paula Costa2, Chieri Kubota3, and Gene A. Giacomelli2. (1) Shantz 429, 1177 E. 4th Street, PO Box 210038, University of Arizona, University of Arizona, Soil, Water and Environmental Sciences, Tucson, AZ 85721-0038, (2) Department of Agricultural and Biosystems Engineering, University of Arizona, Tucson, AZ 85721-0038, (3) Department of Plant Sciences, University of Arizona, Tucson, AZ 85721

Prediction of transpiration is important for design and management of ventilation and irrigation systems within greenhouses in warm, semi-arid regions.  The Penman-Monteith (PM) equation is widely used as a basis for modeling field crop evapotranspiration and has been proposed for predicting plant water use in greenhouses. The equation predicts latent heat flux as a sum of radiation [A (Rn-G)] and aerodynamic [B VPD] terms, where Rn is net radiation, G is soil heat flux, and VPD is vapor pressure deficit in air. The radiation and aerodynamic coefficients A and B depend upon plant and environment factors including temperature, leaf area, and surface and aerodynamic resistances. The purpose of this study was to experimentally determine A and B for tomatoes (Lycopersicon esculentum) grown under controlled VPD and air temperature conditions within a greenhouse at the University of Arizona Campus Agricultural Center in Tucson during spring 2003. Plants were grown under either high VPD (~2 kPa) with day/night air temperature range ~ 27/18 oC or low VPD ( ~0.8 kPa) with temperature range ~ 24/22 oC. Plants were grown in nutrient solution with electrical conductivity (EC) either 2.5 or 8 dS m-1. Transpiration rates from selected plants grown with either nutrient solution were measured using heat-balance based sap flow gauges, however, reliable data were obtained mainly for plants grown in the EC 8 dS m-1 solution. Results indicate coefficients A and B were relatively constant during daytime hours. During low VPD conditions the coefficients were on average A ~ 0.5 and B ~ 0.04 W m-2 Pa-1.  These are comparable to reported values for tomatoes grown in greenhouses in other locations including northern Europe.